This invention generally relates to airfoils.
The invention finds particular utility as an airfoil for use in the main wing of relatively low-speed, general aviation aircraft. A conventional aircraft configuration typically includes two wings connected to a fuselage. Aircraft must operate at a range of speeds and require different wing lift and drag characteristics for different speeds. High lift coefficients are required at lower speeds, as, for example at landing and takeoff. Low drag and lower lift coefficients are desirable for optimum performance at higher speeds. In addition, it is desirable that the airfoil""s lift characteristics not be sensitive to surface roughness caused by the accumulation of foreign matter on the leading edge of the airfoil. Moreover, it is also desirable to have the airfoil exhibit relatively docile stall characteristics. Such characteristics are determined by the shape of the airfoil, which in turn determines the aerodynamic forces exerted on the airfoil as it passes through the air at various speeds and orientations.
For purposes of three-dimensional, aerodynamic efficiency, the chord of an airfoil, or cross-section of a wing, will typically be larger at the root of the span of the wing and will typically become smaller at the tip of the wing. Therefore, a table of coordinates for the geometry of the upper and lower surfaces of an airfoil can remain valid from the root to the tip of the wing, since the coordinates are dimensionless and are provided as percentages of the chord of the airfoil.
Another important parameter for every airfoil or wing cross-section is its operating Reynolds number. The Reynolds number of an airfoil (at a particular span station) is dimensionless and is defined by the following equation: R=cV/xcexd, where R is the Reynolds number, c is the chord of the airfoil, V is the free-stream flow velocity, and xcexd is the kinematic viscosity of the air. Physically, the Reynolds number can be thought of as the ratio of the inertial forces to the viscous forces of air flow over a wing.
Airfoil performance characteristics are a function of the airfoil""s Reynolds number. As the velocity of air over a wing and/or the chord length of a wing decrease, the wing""s Reynolds number decreases. A small Reynolds number indicates that viscous forces predominate while a large Reynolds number indicates that inertial forces predominate.
Another parameter used to describe the aerodynamic performance of an airfoil is its lift characteristics. Normally, the lift of an airfoil or wing is expressed as a lift coefficient, a dimensionless number that measures how effectively a wing converts the dynamic pressure of the flow into a useful lift force. The lift characteristics of an airfoil change significantly as the angle between the airfoil and the apparent wind change. That angle is known as the angle of attack.
Numerous aircraft airfoil designs have been used in general aviation aircraft. Many conventional-aircraft airfoil designs produce diminished lift coefficients if the wings accumulate materials (e.g., insects, dirt or rain) on the airfoil surfaces, especially the leading edge. Such roughness is of concern because the performance characteristics of the aircraft are variable depending on the smoothness of the airfoil surfaces. A number of ways to reduce the sensitivity of aircraft wings to the effects of surface roughness have been devised. One is to induce turbulent flow on the upper surface of the airfoil so that the accumulation of material on the airfoil will not significantly alter air flow or the lift characteristics of the wing. One such technique is disclosed in U.S. Pat. No. 6,068,446 with respect to airfoils for wind turbines.
Thus, one objective of the present invention is to produce an airfoil useful for the main wing of a general aviation aircraft where the airfoil""s maximum lift coefficient has minimal sensitivity to leading edge roughness effects. The primary goal of the invention is to provide an airfoil that efficiently converts the forward velocity of the aircraft into a lift sustaining force. Another object of the invention is to provide an airfoil having a high maximum lift coefficient and low drag. Still another object of the invention is to provide an airfoil having docile stall behavior.
To achieve these and other goals for the present invention there is provided an airfoil shape for the main wing of a general aviation aircraft. In a first embodiment the airfoil has a blunt trailing edge. The airfoil has an upper surface, a lower surface, and a chord line. In such an airfoil, x/c values are dimensionless locations on the chord line and the corresponding y/c values are dimensionless distances from the chord line to points on the upper or lower surface. The values correspond substantially to the following table for the surfaces in the embodiment having a blunt trailing edge:
A second embodiment of the invention is an airfoil shape for the main wing of a general aviation aircraft having a sharp trailing edge. The airfoil has an upper surface, a lower surface, and a chord line. In such an airfoil, x/c values are dimensionless location the chord line and the corresponding y/c values are dimensionless distances from the chord line to points on the upper or lower surface. The values correspond substantially to the following table for the surfaces in the embodiment having a sharp trailing edge:
The airfoil shapes of the present invention are specifically designed for the wing of a general aviation aircraft, although the invention may also have utility in other applications.